1. Field of the Invention
The present invention relates to an elongation measuring device for measuring an elongation in a belt-like member included in a driving mechanism, wound round a rotative driving member and carrying a driven movable object.
2. Description of the Related Art
Some driving device for driving a driven movable object for displacement is provided with a belt-drive mechanism. A belt-drive mechanism employing a timing belt is capable of displacing a movable object at a low cost, in a narrow space without excessively vibrating the driven subject more efficiently than ball screw mechanisms and rack-and-pinion mechanisms.
A conventional driving device includes a drive wheel, a driven wheel, a timing belt and a driving means. The drive wheel and the drive wheel are supported for rotation with their axes extended parallel to each other. A movable object is attached to the timing belt, and the timing belt is extended between the drive and the driven wheel. The driving means drives the drive wheel for rotation. The driving means controls the angular position of the drive wheel to move the movable object to a desired position.
The driving device is unable to move the movable object if the timing belt breaks after doing its term of service. The driving device remains inoperative and cannot perform a series of operations until the broken timing belt is replaced with a new one. Consequently, the working efficiency of the driving device decreases. If the timing belt extended vertically to move the movable object vertically breaks, the movable object will drop by its own weight and may possibly be damaged.
A prior art proposed to solve such a problem is proposed in Patent document 1. A power transmission device mentioned in Patent document 1 includes a drive wheel mounted on a rotating drive shaft, a driven wheel mounted on a rotating driven shaft extended parallel to the rotating drive shaft, and two parallel timing belts extended between the drive wheel and the driven wheel. Power is transmitted to the driven wheel by driving the drive wheel for rotation. The power transmission device is provided with a breakage detecting means for detecting the breakage of the timing belt. A brake is applied to the drive wheel to stop the drive wheel upon the detection of the breakage of either of the two timing belts by the breakage detecting means. Therefore, even if one of the two timing belts breaks, the other timing belt supports the movable object, such as a robot arm, to prevent the robot arm from falling down.
Patent Document 1: JP-A 2000-141274
The prior art power transmitting device is provided with the two timing belts. Therefore, the angular position of the driven wheel cannot be accurately controlled unless the two timing belts operate equally for power transmission and hence the tensions of the two timing belts need to be adjusted periodically to keep the two timing belts equally tensioned. Tension adjusting work for thus adjusting the respective tensions of the two timing belts cannot be easily carried out and requires much time. Although the breakage detecting means is capable of detecting the breakage of the timing belt, the same is incapable of estimating time when the timing belt may break and of preventing the breakage of the timing belts. Such problems arise when belt-like members other than belts are used. Belt-like members other than timing belts are long, flexible members at least wound round a drive wheel and include steel belts, V belts, chains and wires.
The timing belt is subject to flexure failure and the degree of flexure failure increases with time of use and the timing belt lengthens. The driving device provided with the prior art belt-drive mechanism is not provided with any means for measuring an elongation in the timing belt and hence the power transmitting device cannot be controlled on the basis of an elongation in the timing belt. Moreover, time when the timing belt will break cannot be predicted. Since an elongation in the belt cannot be measured, the belt is replaced with a new one before the belt breaks.